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262                          ANCIENT RHINOCEROSES                          CHAP.

are reduced fco five. The lower Incisors are only two. The
sagittal crest is less marked ; the fifth digit is reduced to a tiny
nodule representing the metacarpus. It had a small nasal horn.
There are numerous other details of likeness to modem Rhinoceroses
in this creature, which has only community of descent with them
from the older hornless forms, such as Aceratherium and Caenopus.
In the genus Peraceras the upper incisors are as completely gone
as in the living African Rhinoceroses.

The most ancient rhinoeerotine types 1 are the Hyracodonts and
the Amynodonts. They both, date from the Eocene, and became
extinct in the succeeding Oligocene. Jffyracodon2 (Pig. 134) was
*' an agile, light-chested, and rather long-necked " type, resembling a
Horse in build. There were no horns present, but the hoofs were
more like those of the Horses than of the existing Rhinoceroses.
These animals were apparently plain dwellers and defenceless, which
is held to account for their compact hoofs and outward similarity
to a Hoise. The genus is Oligocene. The dental formula is
If CIPmf Mf.

It is surmised by Professor Scott that the number of dorso-
Inmbar vertebrae was twenty-three or twenty-four. The radius and
ulna are complete and separate bones, but the latter is somewhat
reduced. There are four metacarpal bones, of which, however, the
fifth is much reduced. The animal is only three-fingered. The tibia
and the fibula are distinct, and show no tendencies towards fusion ;
but the fibula is much reduced. There are only three metatarsals
and three toes. Had this line, which is to be regarded as a side
branch of the Rhinoceros stem, not died put, it would probably
have resulted, thinks Professor Scott, in monodactyle  very Horse-
like types. It is later than the next genus to be described,
Hyrachyus, of which it is possibly a descendant. An intermediate
type, Triplopus, appears to bind together Hyracodon and Hy-

In HyrmchA/'&s agrarius the skull is long and narrow, the
facial region being markedly longer than in existing Rhinoceroses.
The masfeoid portion of the periotic bone is v/idely exposed upon
tfce outer face of the skull, which is, as has been said, not the
with the existing genus Rhinoceros. The dentition is the
Entherian dentition of forty-four teeth. The upper